1. Field of the Invention
This invention relates to pressure-actuated depth transducers usually employed with seismic streamer cables at sea.
2. Description of the Prior Art
As is well known, seismic streamer cables used for geophysical exploration at sea, are quite long; two-mile cables are typical. The complete cable is made up of a plurality of sections 100 to 300 feet long. Preferably the streamer cable is towed behind a ship at a desired depth of 20-80 feet. Although great pains are taken to maintain a uniform bouyancy of the cable along its length, lateral variations in water density disturb the uniformity. Accordingly depth transducers are located at intervals along the cable, such as one transducer every thousand feet, to monitor the actual depth of various portions of the cable. Signals from the respective transducers are transmitted back to the towing ship where the signals are converted to a visual display of cable depth at each of the respective depth transducers.
Various types of depth transducers are known. In one known type, a pressure-sensing element moves a soft iron pole piece longitudinally within a coil of wire to vary the inductance as a function of depth. In combination with the pole piece, the wire coil forms the inductive portion of an LC tank circuit. An oscillator aboard the towing ship transmits a continuous sinusoidal signal to the transducer. The LC tank circuit tunes the signal frequency as a direct function of depth. Aboard ship, frequency meters, calibrated in terms of depth, provide a direct depth readout to the equipment operators.
Customarily, the depth transducer is built directly into the cable section. Sometimes the transducer is built into a separate short section (one or two feet long) that is coupled between two conventional cable sections. In any event, each transducer requires its own pair of wires for signal communication with the towing recording ship.
There are certain disadvantages associated with the known depth transducer systems. One objection is the requirement for extra wires in the streamer cable. Since each transducer requires at least two conductors, and if there are ten to twelve transducers, up to twenty-four extra wires are needed in the cable. Another problem is that when a depth transducer must be repaired, the cable section containing it must be completely disassembled to access the transducer. At sea, cable disassembly is a tedious, hazardous task. Finally, it is necessary to test and calibrate the depth transducers before deploying them in the water at the beginning of each operating period. Calibration usually takes place as the seismic cable is being payed out into the water over the stern of the ship from a cable reel. As each depth transducer appears, cable payout is halted and the transducer is exercised. Typically, an inflatable cuff is wrapped around the cable over the transducer. Air at several different known pressures inflates the cuff in much the same fashion as a doctor's blood-pressure measuring device. The depth as shown by the depth readout instrument is compared with the known applied air pressure and the transducer calibration is adjusted accordingly.
A substantial amount of time is lost when the cable payout operation must be halted. In th meantime, with the towing ship idling in the water during calibration, the portion of the cable already payed out drifts aimlessly about on the water's surface and is subject to damage from the propellers of the survey ship and from passing shipping.